MAX1612 Datasheet, PDF (7/12 Pages) Maxim Integrated Products – Bridge-Battery Backup Controllers for Notebooks

Bridge-Battery Backup Controllers
for Notebooks
MICROCONTROLLER
I/O
1M
250k
LRO
MAX1612
MAX1613
BBON
GND
2N7002
Figure 2. Reducing BBON Noise Sensitivity
specifications vary, the counter frequency can be
adjusted to accommodate these variances by adjusting
C_CC. Similarly, the discharging oscillator frequency can
be adjusted with the C_CD capacitor. However, the rate
of bridge battery discharge depends on the DC-DC
converter’s load. Decrementing the charge/discharge
counter is used only to estimate the remaining charge
on the bridge battery. The counter increments (or
decrements) based on CCMD and DCMD logic states.
Note that the net charge must exceed the net dis-
charge to compensate for charging efficiency losses.
Figure 3 shows a typical stand-alone application (see
Design Procedure for details). It reduces the need for
an external microcontroller to manage these functions.
However, if the design requires greater flexibility, a
microcontroller can be used as shown in Figure 4.
DC-DC Converter
The DC-DC step-up converter is a pulse-frequency
modulated (PFM) type. The on-time is determined by
the time it takes for the inductor current to ramp up to
the peak current limit (set via R_BBON), which in turn is
determined by the bridge battery voltage and the
inductor value. With light load or no load, the converter
is forced to operate in discontinuous-conduction mode
(where the inductor current decays to zero with each
cycle) by a comparator that monitors the LX voltage
waveform. The converter will not start a new cycle until
the voltage at LX goes below the battery voltage. At full
load, the converter operates at the crossover point
between continuous and discontinuous mode. This
“edge of continuous” algorithm results in the minimum
possible physical size for the inductor. At light loads,
the devices pulse infrequently to maintain output regu-
lation (V_FB ≥ 2V). Note that the LX comparator requires
the DC-DC output voltage to be set at least 0.6V above
the maximum bridge battery voltage.
Timer Block
The MAX1612/MAX1613 have an internal charge/dis-
charge counter that keeps track of the bridge-battery
charging/discharging process. When CCMD is low and
DCMD is high, the internal counter increments until the
FULL pin goes high, indicating that the counter has
reached all 1s. The maximum counter value is 2²¹.
Additional pulses from the CC oscillator will not cause
the counter to wrap around. In the stand-alone applica-
tion (Figure 3), terminate the charging process auto-
matically by connecting FULL to CCMD. In a micro-
controller application, pull CCMD high. The counter
only specifies the maximum time for full charging; it
does not control the actual rate of charging. CCMD
controls the charging switch, and the resistor at ISET
sets the charging rate.
During the discharging process, drive DCMD low in
order to begin decrementing the counter. When the
counter is full, FULL is high. As soon as the counter
decrements just two counts, the FULL pin sinks current,
indicating that the battery is no longer full. The counter
only indicates the relative portion of the charge remain-
ing. The incrementing and decrementing rate depends
on the maximum charge and discharge times set forth
by charging and discharging rates (see the following
equations for CC and CD). Note that the actual dis-
charging is caused by the input current of the step-up
DC-DC converter loading down the bridge battery,
which is controlled via BBON rather than by DCMD.
The CC and CD capacitor values determine the
upcount and downcount rates by controlling the dis-
charging oscillator frequency. Determine the maximum
charge and discharge times as follows:
C_CC (nF) = 4.3 · t_HRS
C_CD (nF) = 4.3 · t_HRS
where C_CC is the charging capacitor, C_CD is the dis-
charging capacitor and t_HRS is the maximum time in
hours for the process. Choose values that allow for
losses in the battery charging and discharging
process, such as battery charging inefficiencies, errors
in charging current value caused by variable main bat-
tery voltages, leakage currents, and losses in the
device’s internal switch. For charging, use the standard
charge rate recommended by the battery manufactur-
er. The maximum charging current is restricted to
the battery specifications. Consult the battery man-
ufacturer’s specifications. Do not set the charging
current above 10mA.
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